This invention relates generally to a method and means for gauging the fluid contents of a closed container.
In a specific embodiment, this invention relates to a method and means for gauging the level of liquid oxygen contained within a portable, liquid oxygen therapy unit.
There have been developed a number of commercially available liquid oxygen therapy systems, both stationary and portable, to provide supplemental oxygen for persons of impaired breathing ability. Capacity of the portable systems is governed primarily by weight considerations but they are typically sized to provide about three to about fourteen hours of continuous oxygen supply. This capacity is adequate for a person working normal hours and for shopping, visiting and other short trips. In preparation for use, a portable unit is typically filled with liquid oxygen from a larger, stationary home unit.
It is important to the user of such a portable system that he be able to check at will the oxygen level within the unit. As can be appreciated, an unexpected exhaustion of the oxygen supply could create a serious problem or even provoke a medical crisis for the user. With experience, a user can project with some accuracy the time a filled unit will last him. However, portable therapy units typically are arranged for selection of oxygen delivery rates over a rather broad range. If the oxygen delivery rate is changed during the use period, this will throw time projections awry.
One approach that is used to determine the oxygen level within such portable units is to weigh the entire unit, subtract from the weight obtained the empty, or tare, weight of the container, and thus obtain the weight of liquid oxygen within the unit. This technique is cumbersome and requires relatively accurate weight determinations because the tare weight is a substantial proportion of filled container weight. For example, one commercial portable unit has a capacity of about 1.2 lb of liquid oxygen and weighs about 6.6 lb when full. A somewhat larger commercial unit has a liquid oxygen capacity of about 3.8 lb and weighs 13 lb when full.
Another approach that has been taken in the gauging of the contents of a liquid oxygen therapy unit is to provide a spring scale attachable to the unit and calibrated only over that weight range between the tare weight and full weight of the container. Such devices suffer from poor readability and resolution and require frequent adjustment. They are also bulky and frequently are carried as a separate device.
A variety of electrical gages have been proposed and used in the past. These electrical gages have employed sensors within the cryogenic container and have included variable resistance sensors, thermistors, variable capacitance sensors and the like capable of discriminating between liquid and gaseous oxygen. Appropriate circuitry is provided external to the cryogenic container to activate the sensors and to provide a read-out of the oxygen level within the container. As well as being relatively expensive and complicated, such electrical gages require routine battery changes and are prone to maintenance problems.
Although the desirability of providing an integral, simple, accurate and highly readable contents gage for use with liquid oxygen therapy units has long been recognized, no device has yet been developed which fully satisfies these needs.